Flu virus can spread without the aid of coughing/sneezing -- you just gotta breathe. It stays in suspension longer in cold, dry air than in warm, moist air. And it doesn't help that mucus flows more poorly in cold weather.

"...Virtually no lab animals get it the way humans do... The most useful animal has been the ferret... “They’re big, they’re expensive, and they bite,” Peter Palese of Mount Sinai Medical School in New York City told New Scientist.

In 1919, US Army doctors at Camp Cody in New Mexico reported (Journal of the American Medical Association, vol 72 p1056) that the 1918 flu pandemic had killed their guinea pigs – kept at the time for medical tests. “We didn’t know guinea pigs got flu,” says Palese. They are no longer popular lab animals, and no-one had tried them."

Bad news for air travelers:

"“It spread just in the air they exhaled,” says Palese. “Guinea pigs with flu don’t cough or sneeze.”"